Low-temperature permittivity of insulating perovskite manganites

Measurements of the low-frequency (f1000.3em0exkHz) permittivity (ε) and conductivity (σ) at T1500.3em0exK are reported for La_1-xCaₓMnO₃ (0x1) and Ca_1-ySryMnO₃ (0y0.75) having antiferromagnetic, insulating ground states covering a broad range of Mn valencies from Mn³⁺ to Mn⁴⁺. Static dielectric constants are determined from the low-T limiting behavior. With increasing T, relaxation peaks associated with charge-carrier hopping are observed in the real part of the permittivities and analyzed to determine dopant binding energies. The data are consistent with a simple model of hydrogenic impurity levels and imply effective masses m*∕mₑ~3 for the Mn⁴⁺ compounds. Particularly interesting is a large dielectric constant (ε₀~100) associated with the C-type antiferromagnetic state near the composition La0.2Ca0.8MnO₃.